Stereo FM receivers are commonly used in many consumer products. In a conventional stereo FM receiver, an input analog signal is received, down-converted to an intermediate frequency (IF), and digitized before being demodulated to a stereophonic multiplexed (MPX) signal. The resulting MPX signal is composed of three components: (a) a summation signal of left (L) and right (R) channels
  (            L      +      R        2    )at baseband, (b) a pilot tone signal at 19 kHz, and (c) a double-sideband suppressed-carrier (DSBSC) modulated difference signal
  (            L      -      R        2    )at 38 kHz, twice the frequency of the pilot tone signal.
In an FM stereo receiver, the difference component
  (            L      -      R        2    )is extracted from the MPX signal and down-converted to baseband before being added to and subtracted from the baseband summation component
  (            L      +      R        2    )to produce distinct L and R signals for stereo output. Typically, minimal signal processing is required to extract the baseband summation component of the MPX signal. The 19 kHz pilot signal may be recovered by passing the MPX signal through a narrow band-pass filter centered at 19 kHz. This received pilot signal may then undergo frequency doubling to generate a 38 kHz sub-carrier signal. The difference component may be recovered by first passing the MPX signal through a band-pass filter centered at 38 kHz. The resulting signal may then be modulated with the 38 kHz sub-carrier to down-convert the difference component to baseband. The recovered
  (            L      -      R        2    )difference component and
  (            L      +      R        2    )summation component, both in baseband, may then be added and subtracted to generate separate L and R outputs.
In doubling the 19 kHz recovered pilot signal to produce a 38 KHz sub-carrier signal, a strict phase relation between the two signals must be enforced to maintain a separation between the left and right channels in an effort to minimize channel leakage. This is important because channel leakage may have a detrimental effect on the resulting audio qualities. Moreover, the non-ideal nature of the circuit components used in most signal-processing circuitry complicates this phase relation between the pilot tone and sub-carrier signals by injecting noise, such as phase delays, into the waveforms.
This invention relates to systems and methods for producing a 38 kHz sub-carrier signal from a 19 kHz pilot tone signal and for maintaining a phase relation between the two signals in an effort to minimize channel leakage.